An auxiliary power unit (APU) system is often provided on an aircraft and is operable to provide auxiliary and/or emergency power to one or more aircraft loads. In conventional APU systems, a dedicated starter motor is operated during a starting sequence to bring a gas turbine engine up to self-sustaining speed, following which the engine is accelerated to operating speed. Once this condition is reached, a brushless, synchronous generator is coupled to and driven by the gas turbine engine during operation in a starting mode whereupon the generator develops electrical power.
As is known, an electromagnetic machine may be operated as a motor to convert electrical power into motive power. Thus, in those applications where a source of motive power is required for engine starting, such as in an APU system, it is possible to dispense with the need for the dedicated starter motor and operate the generator as a motor during the starting sequence to accelerate the engine to self-sustaining speed. This capability is particularly advantageous in aircraft applications where size and weight must be held to a minimum.
The use of a generator in starting and generating modes in an aircraft application has been realized in a variable-speed, constant-frequency (VSCF) power generating system. In such a system a brushless, three-phase synchronous generator operates in the generating mode to convert variable-speed motive power supplied by a prime mover into variable-frequency AC power. The variable-frequency power is rectified and provided over a DC link to a controllable static inverter. The inverter is operated to produce constant-frequency AC power, which is then supplied over a load bus to one or more loads.
The generator of such a VSCF system is operated as a motor in the starting mode to convert electrical power supplied by an external AC power source into motive power which is provided to the prime mover to bring it up to self-sustaining speed. In the case of a brushless, synchronous generator including a permanent magnet generator (PMG), an exciter portion and a main generator portion mounted on a common shaft, it has been known to provide power at a controlled voltage and frequency to the armature windings of the main generator portion and to provide field current to the main generator portion field windings via the exciter portion so that the motive power may be developed. This has been accomplished in the past, for example, using two separate inverters, one to provide power to the main generator portion armature windings and the other to provide power to the exciter portion. Thereafter, operation in the generating mode may commence whereupon DC power is provided to the exciter field winding.
Patents disclosing systems in which a single dynamoelectric machine is utilized both as a motor to start an engine and as an AC generator which produces AC output power include Lafuze, U.S. Pat. No. 3,902,073, Messenger, U.S. Pat. No. 3,908,161, Hoffman, et al., U.S. Pat. No. 4,093,869, Shilling, et al., U.S. Pat. No. 4,743,777, Dhyanchand, U.S. Pat. No. 4,939,441, Dhyanchand, et al., U.S. Pat. No. 4,947,100, Rozman, et al., U.S. Pat. No. 4,949,021, Dhyanchand, U.S. Pat. No. 4,968,926, Dhyanchand, U.S. Pat. No. 5,015,941, Dhyanchand, U.S. Pat. No. 5,055,700 and Glennon, et al., U.S. Pat. No. 5,068,590.
Operation of the brushless generator as a motor and the subsequent conversion to generator operation can introduce a number of difficulties, particularly with respect to the excitation of the machine. In the generating mode, DC field current is supplied to the main generator portion field winding and a polyphase voltage is generated in the armature winding of the main generator portion. However, during operation in the starting mode, the machine is initially at a standstill so that a DC voltage applied to the exciter field winding will not produce any AC voltage in the exciter armature windings. Thus, at standstill, it is necessary to convert the exciter into a rotating transformer having a stationary primary winding excited from an AC supply so that power can be supplied to the main generator portion field winding. AC power can then be applied to the main generator portion armature winding to set up a rotating magnetic field which interacts with the magnetic field developed by the rotor so that operation in the starting mode can commence.
The above-identified Messenger '161 patent discloses a brushless generator wherein a set of three exciter field windings are connected in a wye configuration during operation in the starting mode such that the stator windings and an exciter armature winding operate as a rotating transformer. After the generator has been brought to speed and is to be operated in the generating mode, the stator field windings are reconnected into a series configuration.
The above-identified Shilling, et al. '777 patent discloses a brushless, synchronous generator having a three-phase AC exciter for use in the starting mode and a DC exciter for use in the generating mode.
The above-identified Hoffman, et al. '869 patent discloses the combination of a single-phase AC exciter for use in the starting mode and a DC exciter for use in the generating mode. The AC field winding is arranged in space-quadrature relation with respect to the DC field winding. By this arrangement, the field which is created when the AC space-quadrature winding is excited will not induce voltages into the DC field winding, which is inactive during the starting mode.
The above-identified Glennon, et al. '590 patent discloses a three-phase AC exciter which receives AC power during the motoring and generating modes.
Each of the foregoing patents discloses a system which requires modification of the existing generator and/or addition of a single-phase or multiphase AC exciter to accomplish operation in the starting and generating modes.
An alternative method of operating a brushless generator in a starting mode has been proposed wherein a single-phase AC supply is connected to the exciter field winding wherein the exciter operates as a rotating transformer. This approach, however, has been found to be not satisfactory inasmuch as the energy transfer across the air gap in the machine is very small, and, therefore, the voltage induced in the exciter armature winding is very low. In order to overcome this problem, a very high AC voltage must be applied to the exciter field winding during operation in the starting mode. This high voltage can cause damage to insulation and other components.
Okada, et al., U.S. Pat. No. 4,841,216 discloses the use of the same exciter field coil for DC excitation during the generating mode and for AC excitation during the starting mode. The problems associated with the need to apply high voltage is solved by switching at higher speeds during the starting mode from high voltage AC excitation to DC excitation. The high voltage AC excitation is obtained by utilizing a transformer as a means to step up AC voltage supplied by an AC power source.